The present invention is related to disc drive testing. In particular, the present invention is related to spin-stand testing.
In a computer disc drive, data is stored on a computer disc in concentric tracks. In many drives, the data is stored using a write head that changes a physical property of the disc. The data is read from the disc by positioning a read head over a desired track and sensing the physical properties of the disc along the track. For example, in a magnetic disc drive, the read head senses magnetic moment boundaries along the disc.
The process for producing a read head or a write head varies depending on the type of read head or write head being produced. Nonetheless, all head manufacturing methods share common characteristics, such as a high degree of manufacturing complexity, small feature sizes, and a susceptibility to manufacturing errors. Because of this, each production method generates a significant number of heads that do not meet specifications.
In order to detect faulty heads accurately, the heads must be tested over a disc surface. In particular, each head must be flown over a disc surface while it performs writing and/or reading operations. Early in the disc drive manufacturing art, this type of testing was performed after the head was assembled in a complete disc drive. However, this proved to be unacceptable because the disc drive had to be rebuilt if the head was found to be faulty.
To overcome the inefficiencies of in-drive testing, the art developed a xe2x80x9cspin-standxe2x80x9d, which allowed the head to be tested before it was placed in a disc drive. In general, a spin-stand includes a spinning disc and a mounting support that supports the head and moves the head to a desired position over the spinning disc. In spin-stands of the prior art, the movement of the head is controlled by two coarse motors that move the head respectively in the xe2x80x9cYxe2x80x9d direction and the xe2x80x9cXxe2x80x9d direction, where movement in the xe2x80x9cYxe2x80x9d direction changes the skew angle between the head and the track and movement in the xe2x80x9cXxe2x80x9d direction changes the radial location of the head over the disc. Most spin-stands include an additional xe2x80x9cXxe2x80x9d coordinate motor for fine positioning. In general, the fine positioning xe2x80x9cXxe2x80x9d coordinate motor is a piezo element that is capable of moving the head in steps less than 10 nanometers long. For many heads, this movement is on the order of {fraction (1/100)} of the width of the head.
During testing, the fine positioning piezo element is used to position a read head at different locations across a data track. A series of tests are then performed on the read head including, for example, error-rate testing, pulse width-fifty testing, track average amplitude. (TAA) testing, and track scan testing.
Under one system of the prior art, the head is positioned using an open-loop servo system that does not measure the position of the head relative to the disc. Instead, the open-loop system passes control signals to the piezo element and calculates the position of the head based how the piezo element is expected to react to those signals. Other spin-stands use closed-loop servo systems to keep the head at a desired position relative to external visible guides. These closed-loop systems measure the position of the head using sensors external to the head and then apply a feedback signal to the servo elements to move the head to a desired position. Note that these closed-loop systems are unable to determine the position of the head relative to a written track on the disc. They are only able to determine the position of the head relative to the visible guides.
In the past, open-loop positioning systems or closed-loop positioning systems with external sensors were sufficient for spin-stand testing because the spin-stand was stable enough to maintain the head in a relatively small area of a track. Thus, testing could be performed on the head without being concerned that the head was moving outside of a desired track location.
With each new generation of disc drives, the density of tracks on the disc has increased causing a corresponding decrease in the width of the read and write heads. Because of this, positioning a head within a track, even on a spin-stand, has become increasingly difficult. To overcome this problem, the art has attempted to make the spin-stand more stable so that the head is less likely to move relative to a track. Although this has improved head stability, it has limited the positioning speed of the spin-stand.
A method and apparatus are provided for testing a head to be used in a disc drive. The method is performed in a spin-stand and includes steps of positioning a head over a radial position on a disc, reading test data from a track at the radial position, reading servo data from the track, and using the servo data to position the head at a desired location within the track. The apparatus for practicing this method includes a disc capable of spinning, a positioning system capable of establishing a position for a head relative to the disc, a servo circuit capable of converting a servo signal into a position value, and a feedback circuit capable of controlling the positioning system based on the position value.